1. Technical Field
This invention relates to a sound system for an automobile, and more particularly, to a sound system capable of dynamically adjusting gain and tone characteristics for a reproduced audio signal in accordance with a vibration level in the automobile.
2. Related Art
Human perception of acoustic signals, such as an audio signal (music, speech, etc.) generated by an audio reproduction system, is affected by the presence of ambient noise. Therefore, audio reproduction systems designed to operate in noisy environments, such as inside an automobile, often contain components that attempt to compensate for perceived degradation of the audio signal caused by noise.
Ambient noise in automobiles, caused by, for example, the engine or the interaction of the tires and the road surface, may not be band-limited but usually has strong components below approximately 200 Hz. This low frequency ambient noise causes problems for automotive audio reproduction systems. First, the volume of the signal as perceived by a listener, commonly referred to as the “apparent volume,” is a function of the noise, and hence, the apparent volume decreases as the noise increases. Listeners may wish to maintain the apparent volume at a constant level, but this is difficult as the ambient noise changes dynamically in an automobile, for example, because of changing road conditions and/or changing automobile speeds. This problem often is referred to as the “apparent volume problem.”
One method of compensating for the apparent volume problem, referred to as the “noise-only-method,” involves increasing the gain of the signal as a function of the ambient noise. This method prevents soft passages from being overwhelmed by the noise. However, the method may be disadvantageous because it varies the gain irrespective of the volume level set by the user, so the method increases the gain in response to increasing noise even for very high volume levels. These increases may result in producing signals that are painfully loud for a listener, harmful to the audio reproduction equipment, or both.
In an automobile, the ambient noise is concentrated in the low frequency range, so signals are masked more in lower frequency ranges than in higher frequency ranges. This causes the signals to lose their desired tone. This problem often is referred to as the “uneven masking problem.”
In general, systems designed to compensate for either the apparent volume problem or the uneven masking problem include some way of estimating the level of the ambient noise. Some automotive audio reproduction systems use a microphone located inside the passenger compartment of the automobile to measure ambient noise. The use of a microphone in the passenger compartment, however, may have several disadvantages. Since the microphone is generally sensitive to all sounds in the automobile, including the signals generated by the audio reproduction system, it may be necessary to filter the signal generated by the microphone to yield a signal representative of the noise.
One method for filtering the microphone output signal involves use a low pass filter to remove higher frequencies where the reproduced audio signal may be concentrated. This method, however, may generate a signal that represents only the sub-audio low-frequency noise, rather than the actual masking noise that may contain higher frequencies. Another method of filtering the microphone signal is to subtract the reproduced audio signal from the microphone signal to obtain a signal representative of the noise. This method may be disadvantageous because it involves the transfer function from the speakers of the audio reproduction system to the microphone. The transfer function may be difficult to determine, and, moreover, may vary dynamically, for example, with changes in the number of passengers in the automobile. Furthermore, using a microphone in the passenger compartment of the automobile may cause increases in the signal in response to speech of passengers, so that the audio reproduction system may attempt to “drown out” conversations.
Other methods implementing complex systems of sensors and digital signal processors also have been used to compensate for ambient noise in an automobile. Many of these complex systems are not economically feasible for use in mass produced automobiles. None of these prior systems provides both an economically viable and satisfactory sounding solution. Hence, it would be desirable to provide a satisfactory sounding and economical automobile sound system that compensates for ambient noise in the automobile.